Scanning probe microscopes (SPMs), such as the atomic force microscope (AFM), are instruments which typically use a sharp tip to characterize the surface of a sample down to nanoscale and even sub-nanoscale dimensions. The term nanoscale as used for purposes of this disclosure refers to dimensions smaller than one micrometer. Sub-nanoscale refers to dimensions smaller than one nanometer. SPMs monitor the interaction between the sample and the probe tip. By providing relative scanning movement between the tip and the sample, surface characteristic data can be acquired over a particular site on the sample, and a corresponding map of the site can be generated. Because of their resolution and versatility, SPMs are important measurement devices in many diverse fields including semiconductor manufacturing, material science, nanotechnology, and biological research.
The probe of a typical SPM includes a very small cantilever fixed to a much larger support (i.e., a “handle”) at its base that is in turn attached to a mounting mechanism for attaching to the positioning mechanism. At the opposite, free, end of the cantilever is a sharp probe tip. The probe tip is brought very near to or into contact with a surface of a sample to be examined, and the deflection of the cantilever in response to the probe tip's interaction with the sample is measured with an extremely sensitive deflection detector such as an optical lever system as described, for example, in U.S. Pat. No. RE 34,489 to Hansma et al., or some other deflection detector such as strain gauges, capacitance sensors, etc. Optical detectors typically comprise a laser spot directed onto the cantilever arm and arranged to reflect off the cantilever arm onto the deflection sensor.
The probe is scanned over a surface using a high resolution three-axis scanner acting on the sample support, the probe, or a combination of both. The instrument is thus capable of measuring the topography or other surface properties or nanomechanical properties of the sample. Cantilever probes can be made from conductive material, enabling measurement of electrical properties.
SPMs may be configured to operate in a variety of modes, including modes for measuring, imaging, or otherwise inspecting a surface, and modes for measuring nanomechanical properties of a sample. In a contact mode operation, the microscope typically scans the tip across the surface of the sample while maintaining a constant probe-sample interaction force. In an oscillation mode of operation, sometimes referred to as tapping mode, the tip of the SPM is oscillated while interacting with the sample at or near a resonant frequency of the cantilever of the probe. The amplitude or phase angle of this oscillation is affected by the probe-sample interaction, and changes in the oscillation are sensed.
As the probe is scanned over the surface of the sample, a probe positioning control system monitors the interaction of the probe with the sample surface such as, for example, deflection of the cantilever (in the case of contact mode), or changes in the oscillation amplitude or phase angle (in the case of oscillating mode). The control system adjusts the probe's position (or average position in the case of oscillating mode) relative to the sample to maintain a constant probe-sample interaction. The position adjustment thus tracks the topography of the sample. In this way, the data associated with the position adjustment can be stored, and processed into data that characterizes the sample. This data can be used to construct an image of the inspected sample's surface, or to make certain measurements of selected surface features (such as, for example, a height of the feature).
The resolution of the data obtained by such probe-based instruments is limited by the physical characteristics of the tip of the probe itself. For surface inspection applications, the tip shape is reflected in the acquired data, a problem that is exacerbated by the fact that SPMs often image very small (e.g., Angstrom-scale) features. As a result, an error in the acquired data results and the corresponding accuracy of the surface image is significantly compromised. Similarly, for nanomechanical property measurement applications, the shape of the probe tip, i.e., its sharpness, substantially affects the force-deformation relationship.
Accordingly, probe tip wear presents a problem that must be addressed in SPM applications. Wear of the probe tip occurs when the probe tip interacts with the sample in the course of conducting measurements. Material can be lost from, and in some cases picked up by, the probe tip, causing changes in the size and shape of the tips. Different types of probes (in terms of shape or materials) have different wear characteristics, and even probes of the same type can wear differently for a variety of reasons. Structurally identical probes can experience different wear trends depending on the nature of the samples being scanned by the probe, the corresponding diverse types of interaction between the probe tip and the samples, and other changing circumstances.
Eventually the probe tip is worn down to a condition where it must be replaced. Replacement of the probe tip involves disengaging the probe from the sample, removing the probe from its mount, and installing a replacement probe onto the mount. The installation of the replacement probe may cause a positional offset between the probe and the probe positioning system to which the probe handle is mounted. This difference in alignment has a magnitude that can be even greater than the size of the sample region being measured by the instrument. Therefore, replacement of the probe presents a practical difficulty of resuming measurement from the point where the measuring process was interrupted. In laser-deflectometer systems, the laser spot needs to be re-aligned to the new position of the cantilever of the replacement probe. The calibration process takes a significant amount of time.
Conventional probe mounting systems use mechanical means for mounting or replacing probe tips. For example, as describe in U.S. Pat. No. 5,705,814 (filed Aug. 30, 1995), probe alignment can be automated to speed up re-calibration procedures. Techniques for error-checking are used after a vacuum-assisted pickup procedure, to calibrate a new probe “clamped to the probe mount using a vacuum clamp, a mechanical clamp, an electrostatic clamp, or other similar clamp.”
Likewise, U.S. Pat. No. 8,925,111 (filed Dec. 4, 2013) describes a chuck with a stacker apparatus configured to mechanically hold several tips. A method is disclosed that prevents contact between the chuck and the head module, which can cause decalibration, and probe tips are held mechanically using a vacuum (as shown, for example, in FIG. 10B).
In addition to mechanical probe mounting systems, some conventional systems incorporate chemical mounting (i.e., adhesives). Chinese Patent No. 100573732C (filed Feb. 28, 2008), for example, describes a cured binder adhesive material composition, capable of curing at room temperature in air. Although such systems maintain a desired geometric arrangement between the driver and the tip, crosslinked chemical compositions are generally more difficult to remove than mechanical ones. Furthermore, some materials can outgas chemicals in vacuum environments, including solvents, plasticizers, or un-crosslinked monomers.